Traffic fatalities decline in states with medical marijuana laws

States that enacted medical marijuana laws, on average, experienced reductions in traffic fatalities, according to a study by researchers at Columbia University's Mailman School of Public Health. Overall, states that passed medical marijuana laws saw an 11 percent reduction in traffic fatalities, on average, after enacting the laws, and had 26 percent lower rates of traffic fatalities compared with states without the laws. The findings are published online in the American Journal of Public Health.

Reductions in traffic fatalities greatly impacted those between the ages of 15 and 44 and were especially striking among those aged 25 to 44 years, a group representing a high percentage of those registered patients for medical marijuana use.

Specifically, the researchers observed an 11 percent reduction of among those aged 15 to 24 years, 12 percent for ages 25 to 44, and 9 percent for those 45 years and older. Operational dispensaries were also associated with a significant reduction in traffic fatalities in those aged 25 to 44 years at 5 percent.

Lacking was strong evidence suggesting reductions among those aged
45 years and older, which is also a group overrepresented in the population of patients registered in state medical marijuana programs. "This finding suggests that the mechanisms by which medical marijuana laws reduce traffic fatalities mostly operate in those younger adults, a group also frequently involved in alcohol-related traffic fatalities," said Julian Santaella-Tenorio, a doctoral student in Epidemiology at the Mailman School of Public Health. In 2004 and 2013, 47 percent of fatally injured drivers with a blood alcohol content of 0.08 or greater were 25 to 44 years old.

The researchers based their findings on data for 1985-2014 from the Fatality Analysis Reporting System, a nationwide census of traffic fatalities information maintained by the National Highway Traffic Safety Administration. The association between medical marijuana laws and traffic fatalities for drivers, passengers, cyclists, and pedestrians was examined for each state enacting the laws. They also evaluated the link between marijuana dispensaries and traffic fatalities. Overall, a total of 1.22 million deaths were attributed to traffic crashes occurring in the 50 states during the study period.

Not all states with medical marijuana laws experienced reductions in traffic fatality rates, and a few states actually experienced increases. In California, after an initial immediate reduction of 16 percent in traffic fatalities and in New Mexico, after an immediate post-law reduction of 17.5 percent, the laws were actually associated with gradual increases in fatality rates. "These findings provide evidence of the heterogeneity of medical marijuana laws and indicate the need for further research on the particularities of implementing the laws at the local level. It also indicates an interaction of medical marijuana laws with other aspects, such as stronger police enforcement, that may influence traffic fatality rates," noted Santaella-Tenorio.

"It is also possible that states with medical marijuana laws and lower traffic fatality rates may be related to lower levels of alcohol-impaired driving behavior in these states," noted Silvia Martins, MD, PhD, associate professor at the Mailman School and senior author. "We found evidence that states with the marijuana laws in place compared with those which did not, reported, on average, lower rates of drivers endorsing driving after having too many drinks. We can also point to other characteristics such as the strength of public health laws related to driving, infrastructure characteristics, or the quality of health care systems, as a partial explanation for these findings."

"The evidence linking medical marijuana laws and traffic fatalities lays the groundwork for future studies on specific mechanisms," said Santaella-Tenorio. "We also expect another area of study will be the association between the laws and nonfatal traffic injuries."

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Co-authors are Christine M. Mauro, Melanie M. Wall, June H. Kim, Katherine M. Keyes, and Deborah S. Hasin--all of the Mailman School of Public Health; Magdalena Cerdá, University of California, Davis; and Sandro Galea, Boston University.

This work was supported by the National Institute on Drug Abuse (grants R01DA037866, R01DA034244, T32 DA031099, K01 DA030449), the New York State Psychiatric Institute; and the National Institute on Alcohol Abuse and Alcoholism (grant K01 AA021511). Dr. Santaella-Tenorio is funded by the J. William Fulbright and the Colciencias doctoral scholarships.

Columbia University's Mailman School of Public Health

Founded in 1922, Columbia University's Mailman School of Public Health pursues an agenda of research, education, and service to address the critical and complex public health issues affecting New Yorkers, the nation and the world. The Mailman School is the third largest recipient of NIH grants among schools of public health. Its over 450 multi-disciplinary faculty members work in more than 100 countries around the world, addressing such issues as preventing infectious and chronic diseases, environmental health, maternal and child health, health policy, climate change & health, and public health preparedness. It is a leader in public health education with over 1,300 graduate students from more than 40 nations pursuing a variety of master's and doctoral degree programs. The Mailman School is also home to numerous world-renowned research centers including ICAP (formerly the International Center for AIDS Care and Treatment Programs) and the Center for Infection and Immunity. For more information, please visit http://www.mailman.columbia.edu.

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